The present invention generally relates to electrochemical cells (i.e., batteries) and, more particularly, to a current collector and seal assembly for sealing the open end of a cell container and providing pressure relief for venting gases when exposed to excessive pressure.
Conventional alkaline electrochemical cells generally include a steel cylindrical can having a positive electrode, referred to as the cathode, which commonly comprises manganese dioxide as the active material. The electrochemical cell also includes a negative electrode, referred to as the anode, which commonly comprises zinc powder as the active material. In bobbin-type cells, the cathode is typically formed against the interior surface of the steel can, while the anode is generally centrally disposed in the can. A separator is located between the anode and the cathode, and an alkaline electrolyte solution simultaneously contacts the anode, the cathode, and the separator. A conductive current collector is inserted into the anode active material to provide an electrical path to a negative outer terminal. An annular polymeric (e.g., nylon) seal provides closure to the open end of the steel can to seal the active electrochemical materials in the sealed volume of the can. An inner cover radially supports the seal. The current collector, inner cover, and seal are typically assembled together to form a collector and seal assembly.
Cylindrical alkaline cells are typically sealed closed by placing the collector and seal assembly in the open end of the steel can and crimping the upper end of the can inwardly and over the outer periphery of the seal to compress the seal. However, electrochemical cells commonly employ electrochemically active materials, such as zinc, which generate hydrogen gas, particularly when subjected to abusive discharge conditions, such as battery reversal, as well as during storage, and sometimes during or following service use. When the can is sealed closed, excessive build-up of high pressure gases within the sealed can may force the crimped closure open and cause damage to the cell and/or the device in which the cell is employed.
One approach to avoiding a potentially excessive build-up of pressure in a cell has been to employ a resealable valve system that periodically releases excessive pressurized gases from within the active cell volume. However, the continued periodic release of pressurized gases may, in some situations, permit the release of electrolyte solution containing salts or other particulate matter, which may foul the resealable valve, and such systems generally require additional costly components. Another approach to avoiding excessive build-up of internal pressure involves employing a sealed membrane that is intended to blow out when exposed to excessive pressure either by puncture or rupture of the membrane itself. A puncture mechanism, such as a spiked member, may be employed to punch a hole in the sealed membrane once the internal pressure reaches a predetermined amount.
A further approach to venting excessively pressurized gases has included the use of a vent formed in the seal which is intended to rupture upon experiencing an excessive pressure build-up in the sealed interval volume of the cell. As an example, U.S. Pat. No. 5,667,912 discloses a current collector assembly having a seal with a thinned portion formed in the seal diaphragm axisymmetrical about a rotation of the central longitudinal axis of the cell. The thinned portion of the seal is intended to shear when the internal pressure exceeds a predetermined pressure threshold, to thereby create a pressure relief vent passage.
While the aforementioned conventional approaches have served to vent high pressure gases in commercial cells, many of these approaches involve complex seal designs which consume a significant amount of volume. Increased collector and seal assembly volume generally results in reduced internal volume available for electrochemically active materials, thus limiting the service performance capability of the cell. Additionally, some conventional venting seals exhibit poor leakage performance. Further, the venting pressure is generally limited in conventional rupture type venting seals due to the difficulty in injection molding the thinned portion of the seal. Accordingly, it is therefore desirable to provide for an electrochemical cell having a simplified, low profile collector and seal assembly that effectively vents pressurized gases at a predetermined pressure, is capable of achieving lower vent pressures, and exhibits enhanced leakage performance.